Method for obtaining polymer/bitumen blends with improved stability and polymer efficiency

ABSTRACT

The present invention is an improvement in a process for producing a blend of bitumen and a block copolymer of a vinyl aromatic hydrocarbon and a conjugated diene comprising mixing from 88 to 99.5 weight percent of bitumen with 0.5 to 12 weight percent copolymer at 135° to 250° C., agitating the mixture for at least 30 minutes, adding 0.01 to 2 weight percent sulfur and maintaining the mixture under agitation for at least 30 minutes. The improvement comprises utilizing high shear mixing at a shear rate of at least 10,000 second -1  to provide said agitation.

BACKGROUND OF THE INVENTION

The present invention relates to a process for blending block copolymersof vinyl aromatic hydrocarbons and conjugated dienes with bitumen andcrosslinking them with sulfur. More particularly, the present inventionrelates to such a process which utilizes high shear mixing.

Asphalt is a common material utilized for the preparation of paving androofing materials and also for coatings such as pipe coatings and tankliners. While the material is suitable in many respects, it inherentlyis deficient in some physical properties which it would be highlydesirable to improve. Efforts have been made in this direction byaddition of certain conjugated diene rubbers, ethylene containingplastics like EVA and polyethylene, neoprene, resins, fillers and othermaterials for the modification of one or more of the physical propertiesof the asphalt. Each of these added materials modifies the asphalt inone respect or another but certain deficiencies can be noted in allmodifiers proposed. For example, some of them have excellent weatherresistance, sealing and bonding properties but are often deficient withrespect to warm tack, modulus, hardness and other physical properties;and some of them improve only the high temperature performance ofasphalt, some only improve the low temperature performance of asphalt,while some lack thermal stability or mixing stability with asphalt.

Since the late 1960s, diene polymer rubbers such as styrenebutadienerubber and styrene-rubber block copolymers such asstyrene-butadiene-styrene and styrene-isoprene-styrene block copolymershave been used to dramatically improve the thermal and mechanicalproperties of asphalts. Practical application of the rubber additionapproach requires that the blended product retain improved propertiesand homogeneity during transportation, storage and processing. Long termperformance of elastomer-modified asphalts also depends on the abilityof the blend to maintain thermal and chemical stability.

Such bitumen/block copolymer mixtures have been crosslinked with acrosslinking agent such as sulfur to provide improved mechanicalproperties. A number of methods for incorporating the sulfur have beenused. One of these involves mixing the bitumen and the copolymertogether and agitating them for a period of time before adding thesulfur. After the sulfur is added, the agitation is continued for aperiod of time. This method is advantageous but has the disadvantagethat the resulting morphology is relatively coarse. A very finedispersion of the polymer would provide better high temperatureproperties. Therefore, it would be advantageous to provide a process forproducing such bitumen/block copolymer blends which have a stable andvery fine morphology. The present invention provides such a process.

SUMMARY OF THE INVENTION

The present invention is an improvement in a process for producing ablend of bitumen and a block copolymer of a vinyl aromatic hydrocarbonand a conjugated diene which comprises mixing from 88 to 99.5 weightpercent of bitumen with 0.5 to 12 weight percent of copolymer at 135° to250° C., agitating the mixture for at least 30 minutes, adding 0.01 to 2percent sulfur to the mixture and maintaining the mixture underagitation for at least 30 minutes. The improvement comprises utilizinghigh shear mixing at a shear rate of at least 10,000 second⁻¹ to providethe agitation.

DETAILED DESCRIPTION OF THE INVENTION

The bituminous component in the bituminous-polymer compositionsaccording to the present invention may be a naturally occurring bitumenor derived from a mineral oil. Also, petroleum derivatives obtained by acracking process, pitch and coal tar can be used as the bituminouscomponent as well as blends of various bituminous materials. Examples ofsuitable components include distillation or "straight-run bitumens",precipitation bitumens, e.g. propane bitumens, blown bitumens andmixtures thereof. Other suitable bituminous components include mixturesof one or more of these bitumens with extenders such as petroleumextracts, e.g. aromatic extracts, distillates or residues, or with oils.

The block copolymers may be produced by any well known blockpolymerization or copolymerization procedures including the well-knownsequential addition of monomer techniques, incremental addition ofmonomer technique or coupling technique. As is well known in the blockcopolymer art, tapered copolymer blocks can be incorporated in themultiblock copolymer by copolymerizing a mixture of conjugated diene andvinyl aromatic hydrocarbon monomers utilizing the difference in theircopolymerization reactivity rates. The manufacture of such polymers isdescribed in U.S. Patent Nos. 3,113,986, 4,226,952, and Reissue 27,145,the disclosures of which are herein incorporated by reference. Thepolymers used herein may be linear polymers, radial polymers havingthree or more arms, and they may also be star polymers having aplurality of arms. The arms referred to are block copolymers of vinylaromatic hydrocarbons and conjugated dienes.

Conjugated dienes which may be utilized to prepare the polymers andcopolymers include those having from 4 to 8 carbon atoms and alsoinclude 1,3-butadiene, 2-methyl-l,3-butadiene(isoprene),2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene and the like.Mixtures of such conjugated dienes may also be used. The preferredconjugated dienes are 1,3-butadiene and isoprene.

Vinyl aromatic hydrocarbons which may be utilized to prepare copolymersinclude styrene, o-methylstyrene, p-methylstyrene, p-tertbutylstyrene,2,4-dimethylstyrene, alpha-methylstyrene, vinylnapthalene,vinylanthracene and the like. The preferred vinyl aromatic hydrocarbonis styrene.

The present invention works with both unhydrogenated and hydrogenatedpolymers. Hydrogenated ones are useful in certain circumstances. Whileunhydrogenated diene polymers have a number of outstanding technicaladvantages, one of their principal limitations lies in their sensitivityto oxidation. This can be minimized by hydrogenating the copolymers,especially in the diene blocks. The hydrogenation of these polymers andcopolymers may be carried out by a variety of well established processesincluding hydrogenation in the presence of such catalysts as RaneyNickel, noble metals such as platinum, palladium and the like andsoluble transition metal catalysts. Titanium biscyclopentadienylcatalysts may also be used. Suitable hydrogenation processes which canbe used are ones wherein the diene-containing polymer or copolymer isdissolved in an inert hydrocarbon diluent such as cyclohexane andhydrogenated by reaction with hydrogen in the presence of a solublehydrogenation catalyst. Such processes are disclosed in U.S. Pat. Nos.3,113,986, 4,226,952 and Reissue 27,145, the disclosures of which areherein incorporated by reference. The polymers are hydrogenated in sucha manner as to produce hydrogenated polymers having a residualunsaturation content in the polydiene block of less than about 20%, andpreferably as close to zero percent as possible, of their originalunsaturation content prior to hydrogenation.

The bitumen/block copolymer compositions of the present inventiongenerally comprise from 88 to 99.5, preferably 92 to 99, weight percentasphalt and 0.5 to 12, preferably 1 to 8, weight percent blockcopolymer. Higher polymer concentrations provide good properties but arenot cost effective. Lower polymer concentrations do not provide acontinuous polymer network necessary for improved properties. These twomaterials are mixed together and then subjected to high shear mixing at135° to 250° C. at a shear rate of at least 10,000 second⁻¹ for a periodof at least 30 minutes in order to achieve a blend morphologycharacterized by a very fine, stable polymer distribution.

After the desired morphology is achieved, sulfur is added in an amountof 0.01 to 2 weight percent and the mixture is maintained under saidhigh shear agitation for at least 30 minutes to achieve completereaction of the sulfur. The irreversible crosslinking reactionstabilizes the dispersion achieved by the high shear mixing. The highshear rate specified above for the agitation of the bitumen/polymerblend is necessary to achieve the desired morphology and to assure astable irreversible fine polymer distribution.

Shear rate is defined as the ratio of the speed of the moving part ofthe mixer or rotor to the gap between the moving part and stationarypart or stator of the mixer. For example, a 3 foot diameter impellerrotating at 100 rpm with a clearance of 1/2 inch would provide a shearrate of about 400 second⁻¹. A Silverson L4R mixer with a diameter of 30millimeters and a gap of approximately 0.1 millimeter rotating at 3000rpm would provide a shear rate of about 50,000 second⁻¹. Shear ratessuitable to this invention may be achieved on a laboratory scale withmixers manufactured by, for example, Silverson or Ross, and on acommercial scale with mixers manufactured by, for example, Siefer orDalworth. A shear rate of at least 10,000 second⁻¹ is required toachieve the desired morphology of the invention.

EXAMPLE

Polymer and asphalt as specified below were mixed at 160° C. for 1 1/2hours using high or low shear mixing. For high shear mixing a SilversonL4R mixer operating at 4000 rpm (greater than 750,000 second⁻¹) wasused. For low shear mixing an impeller low shear mixer operating at 1000rpm (about 000 second⁻¹) was used. After complete mixing of the polymer,sulfur was added and mixed an additional hour. High shear mixing leadsto significant increases in viscosity and ring and ball softening point.Composition and properties of the blends are listed in the Table.

    ______________________________________                                                      A     B       C        D                                        ______________________________________                                        Asphalt - AC5 phr.sup.3                                                                       100     100     100    100                                    Polymer                                                                       1205.sup.1 phr   3       3                                                    KRATON ® D1101.sup.2 phr     3      3                                     Sulfur phr      0.13    0.13    0.13   0.13                                   Shear rate      High    Low     High   Low                                    Penetration,                                                                  decimillimeters                                                               25° C.    99      99      92     92                                    4° C.     41      41      38     38                                    Viscosity                                                                     60° C., poise                                                                          2400    1900    24000  3000                                   135° C., centistokes                                                                   600     460     670    570                                    Ring & Ball     127     119     137    124                                    Softening Point, F                                                            ______________________________________                                         .sup.1 a styrenebutadiene (SB) diblock copolymer                              .sup.2 a linear SBS block copolymer                                           .sup.3 phr  parts by weight per 100 parts of rubber (the block copolymer)

I claim:
 1. In a process for producing a blend of bitumen and a blockcopolymer of a vinyl aromatic hydrocarbon and a conjugated dienecomprising mixing from 88 to 99.5 weight percent of bitumen with 0.5 to12 weight percent copolymer at 135° to 250° C., agitating the mixturefor at least 30 minutes, adding 0.01 to 2 weight percent sulfur andmaintaining the mixture under agitation for at least 30 minutes, theimprovement which comprises utilizing high shear mixing at a shear rateof at least 10,000 second⁻¹ to provide said agitation.
 2. The product ofthe process of claim
 1. 3. The process of claim 1 wherein the bitumenconcentration is 92 to 99 weight percent and the polymer concentrationis 1 to 8 weight percent.
 4. The product of the process of claim 3.